1. Field of the Invention
The invention relates to the field of a methodology and detection system for nonlinear, parallel plate EAM detection, using multiple order sidebands to derive the amplitude of mechanical motion in a MEMS device.
2. Description of the Prior Art
Several important classes of MEMS devices, such as resonators, gyroscopes, and chemical sensors, rely on resonance phenomenon in their operation. In these devices, resonant motion needs to be actuated, sensed, and controlled. Capacitive phenomena are commonly used for transduction in vibratory MEMS devices due to the ease of fabrication, low sensitivity to temperature changes, and other practical advantages. However, conventional capacitive detection methodologies produce a signal proportional to such system parameters as nominal sense capacitance, carrier voltage, and gain of the current amplifier. These dependencies constitute a need to calibrate individual MEMS devices to address fabrication imperfections, and fluctuation of the parameters due to changing environment and aging. A detection technique independent of these system parameters can be of great advantage.
Capacitive detection of harmonic motion is often based on measuring the current induced by the relative motion of the capacitive electrodes. The variable sense capacitor is formed between a mobile mass and anchored electrodes. This capacitor is biased by a certain known DC or AC voltage. The motion induced change in capacitance results in the flow of current, which is converted to voltage. This output voltage detected and related to the motion of the resonant structure. Typically, for the same real estate, parallel plate sense capacitors provide much higher capacitive gradient and thus sensitivity. However, unlike lateral combs, parallel plates generate sense signal that is nonlinear with motional amplitude. Historically, this limited the use of parallel plate capacitive detection to small amplitudes of motion (e.g., sense mode of vibratory gyroscopes).
Electromechanical amplitude modulation (EAM) is a widely used capacitive detection approach. It is based on modulation of motional signal by an AC probing voltage (carrier), and allows for frequency domain separation between the informational signals and feed-through of the driving voltages. Conventional linear EAM can be used for either lateral comb sense capacitors or small displacement parallel plate capacitors. In the linear case only one pair of modulated sidebands exist and calibration of the pick-up signal is needed.
Capacitive detection of motion is commonly used in MEMS; however, conventional methodologies are sensitive to variations in system parameters and require periodic calibration. This disclosure describes an algorithm which is robust to parameters variation. One focus of the disclosure is a parallel plate capacitive detection methodology for real-time measurement of arbitrary amplitude of motion in resonant devices. Unlike conventional methods, the disclosed measurement algorithm does not depend on such system parameters, such as nominal sense capacitance, probing voltage, and trans-impedance gain of the motional current amplifier. A real-time measurement algorithm is formulated and simulated. Feasibility of the developed approach is demonstrated experimentally. The invention is especially valuable for robust capacitive detection and self-calibration in resonant structures, such as gyroscopes, resonant microbalances, and chemical sensors.